I am a leader by default, because nature abhors a vacuum. (Desmond Tutu)

September 29, 2008

Pumpkin spider, Araneus trifolium

From my inbox:

My name is Sarah and I’m 12. I live in Ann Arbor. Recently, my family found a spider
with a body about 3/4 an inch in diameter and with legs it was bigger
than a quarter. It is so huge! It is living in a big web between my
house and some bushes. The web is made up of circles and the spider is only there during evening and night. We think it hides in the bush the rest of the day.
After we found it I did a lot of research and I am pretty sure that it is an Araneus trifolium (PumpkinSpider, shamrock spider).
It has the rings around the legs and is the right size. It is in the
right time of year, has the right type of web, and has the right kind
of behavior.
I don’t know whether it’s male or female. How do I figure out? Are
Araneus Trifolium poisonous? Are they common where we live? Can you
look at the pictures and figure out if I’m right and it is Araneus
trifolium? (they are attached)

and my response:

Hi Sarah,

Thanks for the pictures of your spider. It looks like it is enjoying this time of year!

Pumpkin spiders are very common this time of year, and they are found all over Michigan. Spiders can be very hard to identify from photos, because there are so many species of them, but it seems that you have made a good identification.

which describes both males and females of this species. From that page it seems that females are more often seen later in the season, that they are larger than the males, and that their distinctive leg rings ("annulations") are more distinctive than the males. So I'm guessing that it's a female, but you
should try to measure its size to compare against that reference. (One way to do that in photos is to put something of known size, like a coin, next to the spider when you photograph it.)

which is the Bug Guide, which is hosted by Iowa State University Entomology. They have lots of pictures, as well as an identification service. Their photos match up with your photos so you're again probably confirming that identification.

As to the question of danger from spider bites, the Michigan DNR has a page on insects and spiders:

which talks about the two known spiders of medical importance in Michigan, the Northern Black Widow and the Brown Recluse. Since it's not one of those, you shouldn't worry about it being dangerous. In fact I searched google for the phrase